Although most popular sport in Japan is baseball, definitely the national sport is sumo. Then, there is only one single organization which is administering professional sumo wrestlers, Japan Sumo Association.
Under Japan Sumo Association, there are currently 42 stables; Heya. All professional sumo wrestlers must be the member of one of the stables. They train and most of them live at the stable house together with other wrestlers. The head of stable is called Oyakata, former professional sumo wrestler who has been retired.
Then, nationality of Oyakata must be either a native born Japanese or naturalized Japanese who changed his nationality during his professional sumo career. The sumo wrestlers do not have to be Japanese. Currently there are large number of sumo wrestlers from Mongolia and a few from other nations including from countries in Europe too.
To be continued.